Different species are usually unsuccessful at interbreeding; if they do, the hybrid offspring is usually sterile. In this way, species are kept separate and the diversity of life is maintained. In a study published in PLoS Biology this week , scientists observed that when female worms belonging to the Caenorhabditis genus mate outside their species, they end up with reduced lifespans and fewer offspring than usual. While exploring the possible reasons, they may have uncovered a mechanism underlying the formation of new species.

The primary interest in mating for the male of a species is to pass on its genes to the next generation. For this, its sperm cells have to outperform other sperm cells competing to fertilise the eggs of a female. One way for it to do so is to evolve its sperm to become increasingly motile. In this mad rush to the finish line, these “pushy” sperm cells sometimes end up harming the same female they are trying to fertilise. To survive, females evolve to become increasingly resistant to aggressive sperm. “As a consequence, the feature that is beneficial to males is not optimal or even harmful to females, and vice versa,” explained one of the authors Asher Cutter from University of Toronto via email to this correspondent.

Within species, this “sexually antagonistic co-evolution” is usually in-sync so the sperm cells do not end up harming their own females. However, it may be that the sperm cell of one species has evolved to become more aggressive than can be handled by the females of another species. So when they mate, the foreign sperm ends up breaking through the female’s uterus, invading the ovaries and prematurely fertilising the eggs. Sometimes this can end up sterilising or even killing the female. Cutter and team propose that this explains their observations while interbreeding Caenorhabditis worms.

They tested their theory by mating hermaphrodite worms (self-mating) of one species to males of another species. As expected, the hermaphrodites, being used to the gentler sperm they produce themselves, were found to be especially prone to sterility and death.

“Our studies suggest that fertility problems could be a by-product of sexual conflict among gametes in species with an evolutionary history of intense polygamous mating,” said Cutter. K. Subramaniam, researcher unrelated to the study who studies Caenorhabditis worms at IIT-Madras, recognised the role of sexually antagonistic co-evolution in rapid diversification and evolution of new species. “These findings could provide new mechanistic insights into how the sexual antagonism evolves,” he said via email to this correspondent.

The team now plans to study rare worm hybrids to understand how a single ancestral population “happily” interbreeding split into descendant populations that are averse to doing so. “In this way, we hope to solve the genetic causes of Charles Darwin's 'mystery of mysteries' – how it is that new species form,” said Cutter.